The following values SHOULD be used to initialize the
estimation parameters for a new connection:

(a) RTT = 0 seconds.
(b) RTO = 3 seconds. (The smoothed variance is to be
initialized to the value that will result in this RTO).
The recommended upper and lower bounds on the RTO are known
to be inadequate on large internets. The lower bound SHOULD
be measured in fractions of a second (to accommodate high
speed LANs) and the upper bound should be 2*MSL, i.e., 240
seconds.
DISCUSSION:
Experience has shown that these initialization values
are reasonable, and that in any case the Karn and
Jacobson algorithms make TCP behavior reasonably
insensitive to the initial parameter choices.

RFC 6928 is a proposed update (published June 2011) that says that RTO can be initialized to a lower value (but no lower than 1 sec), and contains an Appendix containing data that justifies 1 sec as a reasonable initial value.